1. Field of the Invention
This invention relates to a polypropylene strap which is used to package articles and to a method of manufacturing the same. The polypropylene strap has a high tensile strength and a low elongation as compared with conventional polypropylene straps of this kind. Furthermore, the polypropylene strap has a high rigidity and a high abrasion resistance, again in comparison to conventional polypropylene straps of this kind.
2 Description of the Prior Art:
Polypropylene is widely used at present for manufacturing regular packaging straps because of its comparatively low price. However, polypropylene straps are subject to longitudinal splitting. In order to prevent such occurrences, the surfaces of polypropylene straps are conventionally subjected to an embossing process.
It is preferable that packaging straps have high tensile strength and not be elongated significantly when stretched. A packaging strap having such properties pervents a packaged article from being deformed. In order to use a packaging strap in an automatic strapping machine, it is necessary that the strap have an excellent adaptability to the machine. In particular, a packaging strap which is to be used in a strapping machine must have a smooth surface, a high rigidity (firmness), a high abrasion resistance, and a flexible, soft surface.
The tensile strength of polypropylene straps is related to the draw (or stretch) ratio (that is, to the rate of orientation of the molecular chains in the straps) of the material. However, in order for the polypropylene strap to have an adequate tensile strength, the draw ratio is limited to around 10. When the draw ratio of the material is increased to over 10, the tensile strength of the resulting strap is put in a plateau-like state in which the tensile strength is decreased, or a so-called "whitening phenomenon" occurs in the strap. As a result, the abrasion resistance of the strap is decreased to a great extent, and the strap cannot be used practically.
There is a known method (disclosed in U.S. Pat. No. 3,394,045) of improving the abrasion resistance of a stretched plastic sheet by subjecting the plastic sheet to flash heating to melt a superficial layer only thereof, thereby putting the molecules in the layer in a non-oriented state. Packaging straps obtained by cutting such a plastic sheet to a predetermined width are now on the market.
Although a strap thus obtained has a flexible surface and an improved abrasion resistance, it does not have improved tensile strength, elongation resistance, and rigidity.